Artemisia annua has been used in China for centuries to treat malaria. Researchers from all over the world have demonstrated that preparations based on Artemisia annua are effective against malaria, even in its most acute forms, caused by infection by the parasite Plasmodium falciparum. The main chemical constituent of Artemisia responsible for this pharmacological activity has been identified as Artemisinin, a sesquiterpene structure containing an unusual peroxide bridge. The structure of Artemisinin is shown below:

Clinical trials conducted with Artemisinin and semi-synthetic derivatives thereof such as Artesunate have confirmed its efficacy in the treatment of malaria. Artemisinin was recently synthesised, but the complexity of the molecule prevents its large-scale synthesis. This is confirmed by numerous research projects designed to synthesise Artemisinin analogues with a simpler structure. More recent studies have preferred to focus on artemisia extracts produced with non-polar organic solvents such as diethyl ether or, more commonly, petroleum ether.
However, the known processes do not enable a satisfactory Artemisinin content to be obtained. Moreover, it is very difficult to remove the traces of solvents to obtain a pharmaceutically acceptable product. The solvents are also used in large amounts in the known methods, and have therefore to be discarded or redistilled.
In recent years, carbon dioxide in the supercritical state has been used to isolate active pharmaceutical substances from plant materials. This technique has been used on an industrial scale for over 20 years to extract flavourings from herbs and spices. The carbon dioxide is removed rapidly and completely by depressurising the extraction apparatus, thus eliminating the problems associated with disposal of solvents and contamination of the finished product with traces of solvent. Moreover, the selectivity of supercritical carbon dioxide is generally greater than organic solvents of a polarity comparable to ethyl acetate or petroleum ether.
Similar procedures can also be successfully employed to extract active components from plant matrices using water in the subcritical state, either alone or mixed with other co-solvents.